Conventionally, in order for surface ships to accurately acquire locations of themselves on a nautical chart and perform an automatic traveling (autopilot) using a ship autopilot controller, ships are generally introduced with a Global Navigation Satellite System (GNSS). GNSS is an inclusive term of “GPS” operated by U.S.A, “GALILEO” operated by Europe, and “GLONASS” opearted by Russia, etc. By introducing GNSS, locations (latitude and longitude) of the ships can be acquired accurately, each location of the ship can be displayed on the nautical chart, and the autopilot using the ship autopilot controller can be performed.
However, with GNSS, a situation may be caused where the location of the ship cannot be calculated and positioning accuracy degrades due to influence of electromagnetic waves emitted by solar flares and/or disturbance waves. The degradation of the positioning accuracy by GNSS will raise a problem of causing a sudden change of a course of the ship under the autopilot using the ship autopilot controller. Moreover, in a case where ships equipped with AISs (universal ship-borne Automatic Identification Systems) share information in the same ocean space through the AISs, a problem may arise that other ships misrecognize the location of the ship.
Therefore, it has been considered to calculate the ship location by using a positioning system for calculating the ship location utilizing buoys floating on the sea surface. For example, JP1987-046383U discloses an art for positioning, upon locking the buoys floating on the sea surface to fixed bases at the sea bottom and providing reflectors that reflect radar waves to the buoys, the current location of the ship by using a positional relation of each buoy with the ship and a positional relation of each buoy with each fixed bases based on a traveling period of time from a transmission of the radar wave from the ship until a reception of the radar wave after reflecting on the reflector (radar reflection wave) and the direction from which the radar reflection wave is received. Further, JP1986-070411A discloses an art for obtaining sufficient navigation calculation accuracy even in the deep water depth range by calculating a location of the system based on a loran radio wave received from a land base station, storing it in a storage, and providing a reference buoy that transmits a location stored in the storage to the ship.
However, the conventional arts represented by JP 1987-046383U and JP 1986-070411A are premised on a situation of reflecting a radar wave by a reflector installed in a buoy, and the positioning cannot be performed when the buoy is not installed with the reflector.
Recently, systems including radar beacons (hereinafter, referred to as “the racon”) that transmit racon response waves to ships in response to radar waves are known. However, the conventional arts are inapplicable to this system because generally, a racon response wave is for displaying a racon mark on a PPI (Plan Position Indicator) screen so that a radar operator can visually confirm a location of the racon and not for calculating the location (latitude and longitude) thereof, and also because it is generally necessary to specify a reception time point of the racon response wave when finding a distance to the racon; however, a racon response wave is not simply a normal radar reflection wave but has a processing delay until a racon transmits the racon response wave after the reception of the radar wave, and it is difficult to specify the start of the reception time point of the racon response wave.
Thus, it has been an important subject to efficiently calculate a location of a ship by using a racon response wave that is transmitted from a racon in response to a radar wave. This subject is also produced similarly when other movable bodies other than ships perform a positioning using response waves transmitted from transponder device(s).
The present invention is made in view of the above situations, and it provides a radar device, a radar positioning system, a radar positioning method, and a computer readable media storing a radar positioning program that can calculate a location of a movable body efficiently using a response wave transmitted from a transponder device in response to a radar wave.